I had doubts about the accuracy of my test kit so I made up a solution of 1 g tartaric acid in 200 ml of water, which is 5 ppt. The test kit uses sulphuric acid so to convert the results to tartaric you simply add 50%. The result came out at 2 ppt sulphuric, so 3 ppt tartaric, 40% lower than the solution sample! I recently tested a young red which seemed to taste too acidic but the result was 3. I now now this to be actually 5, which is still on the low side, as a typical commercial red is around 6. What I took for high acidity was simply the the harshness of the young (2 weeks) wine.